The invention concerns a method for operating a particle sensor for determining a particle content in a gas flow, wherein the particle sensor comprises on the surface thereof a sensor structure for determining a soot load and at least one heating element that is separated from the sensor structure by an insulating layer and with which the particle sensor can be heated up in a regeneration phase, and in doing so a soot load on the particle sensor can be removed, and a heating phase can be carried out with the heating element at least at times before the regeneration phase, wherein a significantly lower temperature compared to the regeneration temperature is controlled in said heating phase, wherein brief drops in temperature as a result of wetting by water can be detected with a temperature sensor integrated within the particle sensor.
The invention further concerns a device, in particular a control and analysis unit, for operating the particle sensor and for performing the method according to the invention.
Particle sensors (PM) are used nowadays for example for monitoring the soot emissions of internal combustion engines and for on-board diagnosis (OBD), for example for functional monitoring of particle filters, for example of a diesel particle filter (DPF). In this case the exhaust gas is led along to the particle sensor downstream of the particle filter through a dual protective pipe construction. What is more, collecting resistive particle sensors are known that analyze a change in the electrical properties of an interdigital electrode structure that is due to particle deposits.
Two or more electrodes can be provided that preferably intermesh with each other in a comb-like manner and that are attached to a ceramic structure. The same are also referred to as interdigital electrodes (IDE) and form the actual sensor element. The electrodes are short-circuited by an increasing number of particles collecting on the particle sensor, which causes a decreasing electrical resistance with increasing particle collection, a decreasing impedance or a variation of a parameter related to the resistance or the impedance, such as a voltage and/or a current. In general, a threshold value, for example of a measurement current between the electrodes, is determined for analysis and the time to reach the threshold value is used as a measure of the collected amount of particles. Alternatively, a rate of change of a signal during the particle collection can be analyzed.
One such resistive particle sensor is described in DE 101 33 384 A1. The particle sensor is made up of two intermeshing comb-like electrodes that are at least partly covered by a capture sleeve. If particles from a gas flow collect on the particle sensor, then this results in an analyzable change in the impedance of the particle sensor, from which the amount of collected particles and hence the amount of particles carried along in the exhaust gas can be concluded.
If the particle sensor is fully loaded, the collected particles are combusted in a regeneration phase using a heating element that is integrated within the particle sensor. For this the ceramic of the sensor element is heated to high temperatures, usually of >600° C. In said regeneration phase the sensor element reacts sensitively to large local temperature changes or to a thermal shock, such as can occur as a result of incident water or drops of water. Such a thermal shock can result in cracks in the sensor element. Therefore, a sensor regeneration can only be requested by the engine controller if there can no longer be water at the sensor installation position according to a calculation of the amount of heat in the engine controller.
The document DE 10 2009 028 319 A1 discloses among other things a method for operating a particle sensor for determining a particle content in an exhaust gas flow of an internal combustion engine, wherein the particle sensor is subjected to a regeneration in regeneration phases at certain time intervals, and in doing so a soot load on the particle sensor is removed. The regeneration is carried out for this after waiting for a sensor release, wherein in the case of a cold start or in the case of a warm start of the internal combustion engine the particle sensor is subjected to drying with a heating element that is integrated therein. Said moisture protection heating is carried out with previously applied fixed times or by means of a drying model.
By strongly subjecting the exhaust system to water from the outside, for example when driving through water, or in the event of the penetration of water into the exhaust system, a marked cooling of the exhaust system can occur. Depending on the exhaust system configuration, said cooling may not be detected by the engine controller, but may result in a risk of thermal shock on the sensor element.
In order to dry the sensor element at the start of a driving cycle, a protective state is provided during the waiting period to a dew point end (DPE), in which the probe is heated to a low constant temperature. At said temperature the sensor element is indeed dried, but still no thermal shock can occur as a result of incident drops of liquid.
In order to ensure the regeneration of a dried sensor element at the starting point, the following conditions must be fulfilled:                sensor operation in the “protective heating before the dew point end” phase for a certain minimum time, typically for example 80 s, and        integration of the amount of heat necessary for the dew point end release in the engine controller.        
With the previously used methods, there is however the possibility that driving through water during a sensor protective heating phase can result in wetting of the sensor element and the moisture no longer being able to be expelled from the sensor element until sensor regeneration is used. The aforementioned conditions would not provide adequate protection if the sensor element is subjected to liquid water while driving through water, for example shortly before the end of the “protective heating before the dew point end” phase. Neither the sensor nor the engine controller would detect said driving through water. Sometimes, a sensor regeneration could be carried out without complete drying out of the sensor element having occurred. Said sensor regeneration could result in damage to the sensor element.
A method for detecting at least one property of a fluid medium is known from DE 10 2010 002 979 A1, wherein a sensor element with at least one heating element and at least one temperature sensor is used, wherein in at least one droplet detection step of the method the sensor element is heated by means of the heating element. For this it is provided that the temperature of the sensor element is detected by means of the temperature sensor, wherein the impingement of a liquid from the fluid medium, in particular droplets, on the sensor element can be concluded from a brief drop in the temperature, in particular a temperature peak. In said document, however, no approach is described whereby the protective heating is adapted as a result of said droplet detection in order to guarantee a dried-out sensor element, for example after driving through water.